Concrete is an important building material for structural purposes and non-structural purposes alike. Concrete, generally speaking, includes cementitious materials and aggregate. There may be one or more types of cementitious material and one or more types of aggregate. Concrete may also include voids and reinforcing materials, such as fiber or steel rod (rebar), wire mesh or other forms of reinforcement. It can have high compressive strength, wear-resistance, durability, and water-resistance, be lightweight, readily formed into a variety of shapes and forms, and be very economical compared to alternative construction materials. The formation process includes the presence of water to permit the cementitious materials to harden and/or hydrate and to form bonds with itself, with any aggregate, and with reinforcing materials. That hydration process, which involves some of the water present being used in those chemical reactions, is well-known and -understood.
Yet the use or value of concrete as a building material may be limited by a number of factors. Those factors pertaining to finished structures and products include: weight, relatively poor tensile strength, ductility, modulus of elasticity, the inability to readily cut, drill or nail, and poor insulating properties. Those factors pertaining to the concrete before setting include: weight, limited flowability (and/or reduced strength caused by adding water to overcome the same), requirement to vibrate or otherwise compact the concrete to limit voids, segregation of aggregate, and the like. Those factors pertaining to the precursor materials or components supplied for use in making concrete structures or products include: cost, weight, and segregation of aggregate and other materials.
Lightweight concretes have been developed to reduce the limiting effect of the weight in finished concrete, among other things. Such lightweight concretes (“LWC”) typically involve replacing some or all of the aggregate in a mix with another form of aggregate that is less dense than commonly-used aggregate. Such aggregate may be known as lightweight aggregate (“LWA”). LWCs often have lower strength (such as tensile, compressive, elastic modulus etc.) than a comparable concrete not using LWA, but may have higher strength-to-weight ratios due to the reduced density of the concrete and the weight for a given structure or product.
A structural LWC is ordinarily considered to have a density between about 90-120 lb/cu.ft. and a compressive strength from 2500 psi to over 8000 psi. These values may be measured by ASTM C567 and ASTM C39, respectively.
A variety of characteristics of the set concrete or its behavior during the manufacturing process can be measured and/or designed into that process. These include tensile strength, compressive strength, elastic modulus, modulus of rupture, plastic density, bulk density, oven-dried density, R-value, coefficient of thermal expansion, crack resistance, impact-resistance, fire resistance, slump, water/cement ratio, paste content by volume, weights, weight-fractions, and ductility.
The amount or characteristics of the LWA used, or the amount of ordinary aggregate replaced by LWA, may be constrained by the need to meet certain minimum characteristics, including but not limited to tensile strength, compressive strength, elastic modulus, flexural strength, or modulus of rupture. Other constraints may include segregation of the LWA within the concrete.
In some cases, other materials are added to the mix or to the precursor materials to improve one or more of the characteristics of the cured concrete or its behavior during the manufacturing process. These may be known as admixtures. Admixtures may be liquid or solid, but are typically liquid unless the mix is to be kept in the dry state, such as for making bagged concrete mix.
It is an advantage for LWC to have a reduced density, higher strengths, higher strength-to-weight ratios, and increased R-value, as well as improved crack resistance, impact-resistance, and fire resistance. Reduction of the density and weight of the concrete offers a variety of advantages, including but not limited to: reduced structure weight, which in turn reduces dead loads in buildings and structures; easier and cheaper transportation and handling of the concrete products, lower transportation costs (equipment/fuel); improved thermal insulating properties, fire resistance, and acoustical properties.